A liquid crystal display device includes a back light unit 1 and a liquid crystal display panel 2 as shown in an exploded cross sectional view of FIG. 10. Further, the back light unit includes a white light source 5, a driving circuit 9 (inverter) to light up the white light source, a housing 3, a reflector 4, a diffuser plate 6, a prism sheet 7, and a reflective polarizer 8.
In the liquid crystal display device, color display is performed by guiding light from this white light source 5 to the liquid crystal display panel side via the back light unit 1, adjusting the amount of transmitted light for every pixel on the liquid crystal display panel 2, and separating the light into any one of red, green, and blue lights for every pixel to transmit.
Generally, for the white light source 5 of the liquid crystal display device, a cold cathode fluorescent lamp (CCFL) is used. FIG. 11 shows a cross sectional diagram of CCFL in the longitudinal direction. As shown in the figure, CCFL has a structure in which the inner wall of a glass tube 11 is coated with a phosphor 12 and both ends of the tube are provided with electrodes 13. Further, mercury and a rare gas (argon or neon) are sealed in the tube as a discharge medium 14.
CCFL for use in this kind of back light has a very long and narrow characteristic shape that is different from, for example, a fluorescent lamp for interior illumination, and in the case of a 32-inch liquid crystal display device, the diameter of the tube is ca. 4 mm and its length is ca. 720 mm.
Illumination of this CCFL is carried out by applying high voltage to the electrodes on both ends as is known. Electrons released from the electrode by applying voltage excite mercury, and the excited mercury radiates ultraviolet light when returning to the ground state. The phosphor is excited by this ultraviolet light to radiate visible light to the outside of the tube.
The phosphor 12 provided in CCFL is formed by mixing powders of a blue phosphor that emits blue light (main emission peak wavelength of approximately from 400 to 500 nm), a green phosphor that emits green light (main emission peak wavelength of approximately from 500 to 600 nm), and a red phosphor that emits red light (main emission peak wavelength of approximately from 600 to 650 nm) in a certain weight ratio so as to give a predetermined white color.
Typically, a blue phosphor BaMgAl10O17:Eu2+, a green phosphor LaPO4:Tb3+,Ce3+, and a red phosphor Y2O3:Eu3+ are used. According to the common notation for phosphor material, the front of “:” mark represents a host material composition while its back represents a luminescence center, implying that part of atoms in the host material is replaced by the luminescence center. For example, in the red phosphor Y2O3:Eu3+, Y2O3 is the host material, and part of yttrium (Y) is replaced by europium (Eu). Hence, Y2O3:Eu3+ may be represented by (Y,Eu)2O3.
The visible light radiated from CCFL passes through the diffuser plate 6, the prism sheet 7, and the reflective polarizer 8 that are arranged directly on CCFL and enters into the liquid crystal display panel 2 as shown in FIG. 2. To enhance the efficiency of utilization of light from CCFL, the reflector 4 is placed on the backside of CCFL, and the light reflected from this also enters into the liquid crystal display panel 2.
On the other hand, the liquid crystal display panel 2 has a cross sectional structure shown in FIG. 16. That is, a pair of glass substrates 21 (21A and 21B) opposite to each other are arranged, alignment layers 23 are coated on the inner surfaces of the substrates, and a liquid crystal 24 and color filters 25 (red color 25A, green color 25B, and blue color 25C) are sandwiched between the substrates.
The glass substrates (21A and 21B) are held by spacers 26 placed therebetween. Polarizing plates 22 (22A and 22B) are placed on the outer surfaces of the substrates 21. The liquid crystal 24 is in a uniform alignment state by the alignment layers 23 and driven by applying voltage to a group of electrodes formed for every pixel (not shown in FIG. 16). When voltage is applied, the liquid crystal rotates in response to an electric field generated by the voltage, thereby bringing about a change in the refractive index of the liquid crystal layer to adjust the amount of transmitted light. The color filters 25 separate white light W from a back light unit into red light R, green light G, and blue light B for every pixel and transmit any one of the color lights.
There are various display modes depending on initial alignment of liquid crystal and driving thereof. Typical display modes include in-plane switching (IPS) mode, vertically aligned (VA) mode, optically compensated bend (OCB) mode, and twisted nematic (TN) mode.
In this way, the color display of the liquid crystal display device is performed by adjusting the amount of transmitted light from the white light source 5 provided in the back light unit for every pixel as well as separating the light by the color filters 25 that transmit any one of red, green, and blue lights for every pixel on the liquid crystal display panel 2. It should be noted that this type of related art includes, for example, Patent Document 1; U.S. Pat. No. 4,345,249 and Non-patent Document 1; IDRC'95 (Asia Display 95), p. 577 (1995).